Advent Therapeutics is a company that develops new therapies for neonatal, pediatric, and Orphan Drug ?niche? markets to address serious unmet medical needs. Our team proposes early stage development of a new treatment for hereditable pulmonary arterial hypertension (hPAH), a rare, hereditary disorder that involves progressive elevation of pulmonary arterial pressures, right heart failure, and death. There are no effective treatments. hPAH is often caused by premature termination codons (PTCs) in the mRNA encoded by nonsense mutations in the type II bone morphogenetic protein receptor (BMPR2) gene. Certain antibiotics possess the capacity to induce ?readthrough? of PTCs and thus produce a full-length protein, but are unacceptably toxic. To obviate this toxicity, we first performed a high-throughput screen on a 70,000-compound small molecule library, using a readthrough assay that indicated hits. We derivatized and patented hits, achieved in vitro, ex vivo, and in vivo proof-of-concept using PTCs in disease-causing genes other than BMPR2, and published these results. We next selected a molecule named GJ103 from among the derivatized compounds for further development, based on readthrough efficiency and favorable physical and solubility characteristics, and began testing in hPAH, using blood outgrowth endothelial cells obtained from patients with hPAH that contained disease-relevant PTCs in BMPR2. GJ103 treatment induced significant expression of BMPR2 protein. Further testing with mouse cells corroborated the results with human cells. Next, we collaborated with the Morell lab (University of Cambridge, UK) to test GJ103 in mice they created that develop hPAH by virtue of one of two clinically-relevant nonsense mutations in Bmpr2 (Bmpr2+/R899X and Bmpr2+/R584X mice). After performing preliminary dose-finding studies, GJ103 induced up to half of wild-type BMPR2 protein levels in vivo. Preliminary acute and chronic toxicity testing failed to show evidence of toxicity. BMPR2 protein induced by GJ103 demonstrated its functional effects in an LPS permeability assay in vitro and in vivo, and confocal microscopy studies indicated the protein appeared to traffic properly to the cell surface of endothelial and smooth muscle cells. Downstream upregulation of BMP signaling intermediaries (SMADs and phosphoSMADs) and BMP pathway gene targets (Id1, Id2, and VE- cadherin) indicated functional activation of BMP signaling. Here we propose to test the critical question for using GJ103 to treat hPAH: can GJ103-induced expression of BMPR2 prevent or slow development of hPAH in Bmpr2+/R899X or Bmpr2+/R584X mice if treatment is initiated early enough? In Aim 1, we will collaborate with the Soban lab (UCLA) to test whether GJ103 can induce expression of BMPR2 sufficient to slow or halt development of hPAH in mice. Dependent variables include testing right heart hemodynamics and remodeling using echo and direct catheterization in Bmpr2+/R899X and Bmpr2+/R584X mice treated with GJ103. Aim 2 proposes concomitant expression analyses with the Rehan lab (LA BioMed) of BMPR2 protein, which should be elevated if GJ103 is successful. Our studies might represent the first hope for individuals suffering from hPAH.